1. Field of the Invention
This invention lies in the field of control of electrohydraulic devices and similar apparatus. More particularly, it lies in the field of the electrohydraulic vibrators which are used to generate seismic signals in the earth. Still more particularly, it concerns a digital apparatus to generate both a reference and a drive sweep signal for the vibrator, and includes means to change the relative phase of these two sinusoidal signals.
2. Description of the Prior Art
In the prior art, there have been a number of designs of phaselock systems by means of which a seismic vibrator can be phaselocked or synchronized with a reference sweep signal. These have generally been analog devices, which are slow in response and in correcting the phase of the drive. Furthermore, they are prone to overdrive or overcorrect, which results in hunting of the driven apparatus. Such analog systems further provide very poor phaselock at low frequencies, and poor recovery of control on the occurrence of transient interruptions or variations in the output of the device.